This invention relates to electric switching devices and, more particularly, to solid state power controllers for use in DC circuits.
Relatively high voltage DC power distribution systems are being developed for new aircraft and space electric power systems. These systems present challenging requirements for solid state power controllers that will be used for controlling and protecting loads and load buses beyond what is provided for by present state-of-the-art switchgear, either solid state or electromechanical. These power controllers must provide low power dissipation and low switch voltage drop and be capable of fast, accurate current limiting. For certain applications where the direction of current flow is subject to change at unpredictable times, bi-directional control of load and source current flow is required.
This invention seeks to provide efficient and reliable solid state power controllers having a low voltage drop and fast response time.
SUMMARY OF THE INVENTION
Solid state power controllers constructed in accordance with this invention include an output circuit stage having a first transistor connected between a pair of output terminals. The collector of the transistor is connected to one end of a series circuit of two diodes which form part of the transistor drive circuit. A bias current supply is connected to supply a DC bias current to the diode circuit branch and the drive also includes means for electrically connecting the other end of the diode circuit branch to the base of the transistor.
In an alternative embodiment of this invention, which is applicable to bi-directional power systems, the output circuit stage includes a second transistor connected in series with the first transistor and a second pair of diodes which are reverse series connected in a circuit branch that is connected in parallel with the two output transistors. A common connection point between the transistors is connected to a common connection point between the second pair of diodes.
In both of the above single polarity and bi-directional circuits, current in the output stage is sensed and the DC bias current may be adjusted to be proportional to the output stage current. This provides fast response time when current limiting operation is required.